1. Field of the Invention
The present invention relates to a method of manufacturing a mount structure having a variety of electronic parts mounted on a printed circuit board (hereinafter called the PCB) by soldering, and to a mount structure, and more particularly, to a method of manufacturing a mount structure having electronic parts soldered on at least one side of a PCB by reflowing, and to a mount structure.
2. Description of the Prior Art
Conventionally, soldering has been used for mounting a variety of electronic parts on a PCB. In the following, a method of mounting electronic parts using soldering will be described below with reference to the drawings.
Referring first to FIGS. 1A-1F, description will be made on an example of double-sided reflow process for soldering electronic parts on both sides of a PCB by reflowing.
First, solder paste 81 is printed on a land (not shown) of PCB 80 using a printing mask (not shown) which is provided with an opening only at a position corresponding to the land (FIG. 1A).
Next, surface mounted part 82 such as a chip part, QFP (Quad Flat Package), SOP (Small Outline Package) or the like is loaded on solder paste 81 printed on the land (FIG. 1B).
Then, solder paste 81 is melted by passing PCB 80 having surface mounted part 82 loaded thereon through a high temperature furnace to solder leads of surface mounted part 82 with a copper foil of PCB 80 (FIG. 1C). Through the respective previous steps, surface mounted part 82 has been soldered on one side of PCB 80 by reflowing.
Next, PCB 80 is turned upside down, and solder paste 83 is printed on the other side of PCB 80 on which no electronic part has been mounted (FIG. 1D), and surface mounted part 84 is loaded on printed solder paste 83 (FIG. 1E) by similar steps as those illustrated in FIGS. 1A, 1B. Subsequently, surface mounted part 84 is soldered by passing PCB 80 through the furnace in a manner similar to the step illustrated in FIG. 1C (FIG. 1F).
Electronic parts which cannot withstand the high temperature in the furnace for the aforementioned reflow processing are manually soldered after the reflow processing is terminated.
Referring next to FIG. 2, description will be made on an example of a reflow/flow composite process for soldering electronic parts on one side of a PCB by reflowing and then soldering electronic parts on the other side of the PCB by flowing.
First, in steps similar to those in FIGS. 1A, 1B, solder paste 81 is printed (FIG. 2A), and surface mounted part 82 is loaded on printed solder paste 81 (FIG. 2B) on one side of PCB 80. Then, PCB 80 is passed through a furnace to solder surface mounted part 82 in a step similar to that in FIG. 1C (FIG. 2C). Through the respective previous steps, surface mounted part 82 has been soldered on the one side of PCB 80 by reflowing.
Next, from the one side of PCB 80 on which surface mounted part 82 is mounted, leads of electronic part 85 (hereinafter called the throughhole part 85) are passed through throughholes of PCB 80 to load throughhole part 85 (FIG. 2D)
Subsequently, PCB 80 having throughhole part 85 loaded thereon is passed above a solder bath within the furnace. Then, melted solder 86 is sprayed from the solder bath toward leads of throughhole part 85 on the other side of PCB 80 to solder the leads of throughhole part 85 with a copper foil of PCB 80 (FIG. 2E).
Electronic parts which cannot withstand the high temperature in the furnace for the aforementioned reflow processing, electronic parts which are mounted through the flow processing with difficulties are manually soldered after the flow processing is terminated.
The conventional method of mounting electronic parts described above has generally used an Sn—Pb based solder. However, this Sn—Pb based solder contains Pb, a poisonous heavy metal, which adversely affects the earth environment if devices are not appropriately disposed after use. For this reason, in recent years, a Pb-free solder (Pb-less solder), which does not contain Pb, is desirably used for solving the foregoing problem to obviate the environmental pollution.
Generally, an Sn—Ag based solder is widely known as the Pb-free solder. Since the Sn—Ag based solder exhibits relatively stable characteristics, it can also ensure the reliability as high as before when it is used for mounting electronic parts as a substitute for the Sn—Pb based solder. However, the Sn—Ag based solder has a melting point at slightly lower than 220° C. which is higher than the melting point of the Sn—Pb based solder, i.e., approximately 183° C. This higher melting point makes it difficult to utilize a mounting apparatus and a mounting method for use with the Sn—Pb based solder as they are. Particularly, while typical electronic parts withstand approximately 230° C. of temperature, such electronic parts may be heated in some cases to 240° C. or higher if the Sn—Ag based solder, which has the melting point as high as 220° C., is melted within the furnace for soldering. Therefore, the use of the Sn—Ag based solder for mounting a variety of electronic parts implies a problem that the electronic parts are required to withstand higher temperatures.
Another Pb-free solder, unlike the Sn—Ag based solder having a high melting point, is an Sn—Zn based solder, the melting point of which is slightly lower than 200° C., so that if this Sn—Zn based solder is used to mount electronic parts, conventional facilities and electronic parts can be used as they are.
However, the aforementioned Sn—Zn based solder has a problem of high susceptibility of Zn to oxidization, and poor wettability, as compared with the conventionally used Sn—Pb based solder. This causes difficulties in ensuring the soldering quality and reliability similar to before if electronic parts are mounted by the Sn—Zn based solder using conventional facilities and mounting method.
As a method of improving the wettability of the Sn—Zn based solder, it is generally known to contain Bi in the Sn—Zn based solder, and techniques have been disclosed somewhere for performing reflow processing using such an Sn—Zn—Bi based solder.
However, when the Sn—Zn—Bi based solder is reflowed using electronic parts having leads applied with the currently most general Sn—Pb plating, the conventional reflow processing experiences segregation of Sn—Zn—Pb and Sn—Pb—Bi, resulting from the reaction of Pb which is a main component of the Sn—Pb plating with Sn—Zn—Bi which are components of the Sn—Zn—Bi based solder, in regions around interfaces between lands and solders, leading to the formation of low-strength/low-melting point alloy layers. Consequently, the low-strength/low-melting point alloy layers in these interface regions adversely affect the mounting quality and reliability.
Particularly, in the second reflow processing in the aforementioned double-sided reflow process, and in the flow processing in the reflow/flow composite process, the segregation of low-strength/low-melting point alloys such as Sn—Zn—Pb, Sn—Pb—Bi and the like advances more on the side on which electronic parts have been mounted first. Therefore, the second reflow processing in the aforementioned double-sided reflow process and the flow processing in the reflow/flow composite process experience a phenomenon that the low-strength/low-melting point alloys alone remain unsolidified on interfaces between lands and solders in a cooling step for cooling down the solders. This causes a higher susceptibility to partial delamination or complete failure in bonding of electronic parts in the interface regions when a PCB is bowed or twisted. This phenomenon is prominently found in leads that are positioned at four corners of electronic parts such as QFP, SOP and the like, which are highly susceptible to stresses caused by a bowed or twisted PCB.